Spinal fixation systems may be used in surgery to align, adjust and/or fix portions of a spinal column, i.e., vertebrae, in a desired spatial relationship relative to each other. Many spinal fixation systems employ a spinal rod for supporting the spine and for properly positioning components of the spine for various treatment purposes. Vertebral bone anchors, comprising pins, bolts, screws, and hooks, engage the vertebrae and connect the supporting spinal rod to different vertebrae. Spinal rods can be anchored to specific portions of the vertebra. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone.
Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a threaded shank that is adapted to be threaded into a vertebra, and a head portion having a spinal fixation element-receiving portion, which, in spinal rod applications, is usually in the form of a U-shaped slot formed in the head portion for receiving the rod. A set-screw, plug, cap or similar type of closure mechanism is used to lock the rod into the rod-receiving portion of the pedicle screw.
In conventional spinal surgery, first, anchoring devices are attached to vertebra, then a spinal rod is aligned with the anchoring devices and secured. For example, for conventional pedicle screw assemblies, first the engagement portion of each pedicle screw is threaded into a vertebra. Once the pedicle screw assembly is properly positioned, a spinal fixation rod is seated in the rod-receiving portion of each pedicle screw head. The rod is locked into place by tightening a cap or similar type of closure mechanism to securely interconnect each pedicle screw to the fixation rod. This type of conventional spinal surgical technique usually involves making a surgical access opening in the back of the patient that is almost as long as the length of the spinal rod to be implanted. Because exact placement of the screw assemblies depends on a patient's particular bone structure and bone quality, the exact position of all screw assemblies cannot be known until after all the assemblies are positioned. Adjustments, such as bending, are made to the spinal rod to ensure that it aligns with each screw assembly.
Recently, the trend in spinal surgery has been moving toward providing minimally invasive surgical (MIS) devices and methods for implanting spinal fixation devices. An example of a minimally invasive method is a rod-first method that includes inserting a spinal rod through a first incision and positioning the spinal rod along a patient's spinal column adjacent to one or more vertebra. After the spinal rod is inserted, one or more bone anchors are inserted adjacent to the spinal rod, each through a separate incision. After a spinal bone anchor is inserted and anchored in bone it is connected to the spinal rod. The rod-first method is a minimally invasive technique in which the bone anchors are inserted adjacent to the rod, after rod insertion, then connected with the rod, as opposed to a conventional surgical technique in which the anchors are inserted first, then the rod is placed such that it lies over the anchors.
Thus there is a need to be able to insert bone anchors, and more particularly connectors for connecting the bone anchors to the rod using minimally invasive techniques. Accordingly, what is needed is a rod anchoring system that can be delivered percutaneously to an implant site though a small access port such as a cannula.